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| Titel |
Real-time in-situ tracking of Lagrangian coherent structures in a coastal region |
| VerfasserIn |
Francesco Nencioli, Francesco d'Ovidio, Andrea M. Doglioli, Anne A. Petrenko |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048830
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| Zusammenfassung |
| In recent years Lagrangian techniques have become increasingly important for the analysis of
horizontal mixing and transport properties in the oceans. In particular, the Finite Size and
Finite Time Lyapunov Exponent methods (FSLE and FTLE respectively) have been
frequently applied to identify Lagrangian coherent structures (LCSs: i.e. stable and unstable
manifolds of hyperbolic points) from satellite derived velocity fields and from numerical
simulation results. LCSs can be reliably detected (with errors of ~ 10s km) in the open ocean
from satellite altimetry, and several studies have shown a tight correlation between these
structures and advected tracers. However, as altimetry missions are primarily designed to
operate over the open ocean, altimetry data are less reliable closer to the coast,
limiting the accuracy with which LCSs can be located in coastal regions. This is a
severe problem that affects our understanding and monitoring capabilities of neritic
ecology and biogeochemistry, as well as of dispersal of urban and river discharged
pollutants.
The LAgrangian Transport EXperiment (LATEX, 2008-2011) was designed to study the
anticyclonic mesoscale eddies which form on the continental shelf in the western part of the
Gulf of Lion, and their influence on mixing and cross-shelf exchanges. During the Latex10
campaign (September 1-24, 2010) we attempted a mapping of coastal LCSs with an adaptive
strategy that combined satellite data, ship-based Acoustic Doppler Current Profiler
(ADCP) measurements, and iterative Lagrangian drifter releases. The position of the
LCS tangle was first guessed by combining altimetry derived FSLE with satellite
images of sea surface temperature and chlorophyll concentration. Three arrays
of drifters (14 drifters total) were then released at interval of few days to obtain
in-situ estimates of the structure. The deployment position and configuration of
each array was chosen on the basis of the outcome of the previous launch. Drifter
trajectories were integrated with ADCP mapping to obtain a synoptic picture of the
LCS.
This iterative process allowed to successfully localize and track the repelling and
attracting LCSs present in the region for about 12 days from September 12 to 24.
Their intersecting point was characterized by a slow south-westward migration
(~ 1-3° in about 6 days, corresponding to ~ 5 cm s-1). Our analysis showed that
cross-shelf exchanges were constrained by the presence of a corridor ~ 30 km wide,
roughly parallel to the coast, maintained by the attracting LCS. Future work will
include a quantification of the flux within this corridor by combining the surface
transport information with the temperature vertical profiles also collected during
the cruise; furthermore, in order to obtain better estimates of LCS from FSLE,
methods to improve satellite derived velocity fields in coastal regions will also be
explored. |
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